Conventionally, as a method for growing a single crystal of an oxide or the like, a method is known in which raw materials are melted under heating in a crucible, thereafter a seed crystal is brought into contact with the raw material melt and a round bar-like single crystal is pulled up with rotating this seed crystal to grow a single crystal. This pulling method is used for the growth of various single crystals as a method which can grow a large-diameter crystal in an efficient manner.
Also, for example, a method (Kyropoulos method) in known in which raw materials are melted under heating in a crucible, thereafter a seed crystal is brought into contact with the raw material melt and the temperature below liquid level is slowly lowered to precipitate and grow a crystal.
However, in the conventional methods of growing a single crystal by bringing a seed crystal into contact with a raw material melt, there is the problem that when the viscosity of the raw material melt during growth at the required temperature is high, the flow of the raw material melt in the crucible is retarded, causing nonuniformity in temperature, the degree of supersaturation or the like, leading to the result that the quality of the crystal tends to be lowered.
For example. CsLiB5O10 (CLBO) and the like as non-linear optical crystals attract attention as those for the generation of high output ultraviolet laser light and are desired to have excellent performances and qualities such as very high resistance to damages by lasers, very high optical loss and high uniformity. However, since it is a borate type crystal its melt has a high viscosity which makes it difficult to grow a high quality and high performance single crystal it has been confirmed that in actual measurement, the viscosity of a CLBO solution of a self flux composition is as high as about 1000 cs (centistokes) at a growth temperature, around 840° C.
For example in the growth of a single crystal by means of the rotation of a seed bar in the CLBO cooling method, the distribution of temperature in the raw material melt is not good an shown in FIG. 7 and the growth of a crystal is rapid and therefore there is a restriction inevitably on the growth of a high quality and high performance crystal.
In view of this situation, it is an object of the present invention to solve the problems in prior art as mentioned above and to provide an improved new method which can grow a high quality and high performance single crystal in the case of using even a highly viscous raw material melt and also to provide an apparatus for practicing the method.